The Atom was Acorn's first 'serious' kit machine after the System 1, and only came about because of Chris Curry's belief in the system he'd wanted to produce whilst at Science of Cambridge. Here's a quote from him at the time, with my additions in square brackets:
I had to push hard for the Atom. I did the development separately with one chap [Nick Toop]. To save money, the case for the Atom was designed not as a computer but as a keyboard for the Systems [1,2,3,4,5]. We asked the industrial designer [Roger Wilson] to make something which was low-key, not too flashy. He produced the Atom."
RAM for the Atom was a staggering 2K, expandable to the dizzy heights of 128K with an expansion board. Processor-wise the 6502 was chosen over the Z80 used by Sinclair because Acorn felt it was a faster chip, contrary to popular belief at the time. The BASIC interpreter was a development of the variant used when they were a fledgling company writing process control systems and so was built for speed, not comfort. This caused a flurry of criticism because most of the other manufacturers were using a form of Microsoft BASIC.
Launched in 1980, the Atom was an immediate success despite the critics' misgivings about the BASIC. It was available as either a kit for approx. £120 or fully assembled for £150 which meant both hobbyists and people new to computers could dive straight in and start working, though if the machine was THAT popular it doesn't really explain their apparent rarity these days, unless people just chucked 'em when they upgraded.
Several add-on kits were available for the Atom from both Acorn and 3rd parties, including a hi-res colour board, memory boards a-plenty, ECONET, software on ROM and a keyboard to allow disabled people to use the machine.
Because my Atom came with a massive home-made PSU containing 2 of the biggest capacitors I've seen I didn't dare plug it in 'cos I didn't want to be in the same room as them if they decided to pop :) A chance finding on eBay led me to be fortunate enough to meet Nick Toop himself at his office, and he kindly gave the machine a full health check. Good job he did 'cos if I'd plugged an unregulated PSU into it (eg Spectrum/ZX81) I'd have fried the internals - the previous owner had fitted a switch internally that shorted out the regulators! Thanks Nick!

Thumbnails:
All images and text © Adrian Graham 1999-2010 nless otherwise noted using words.